23-07-2012, 04:05 PM
Mechanical transmission and its elements
Mechanical transmission and its elements.docx (Size: 18.67 KB / Downloads: 30)
Elementary transmissions for transmitting rotary motion:
The important elementary transmissions which are used for transmitting rotary motion from one shaft to another are as given below:
(a) Gear Transmission:
In a gear mechanism, the rpm of the driven shaft is determined as
n2=n1. Z1∕ Z2
where n2=rpm of the driven shaft
n1=rpm of the driving shaft
Z1=no. of teeth of driving gear
Z2=no. of teeth of the driven gear
The ratio Z1∕ Z2 is known as the transmission ratio of the gear drive and is constant for a particular gear pair.
Rotation is transmitted between parallel shafts by means of spur, helical and herringbone gears. Spur gears have teeth parallel to the axis of rotation, while in helical gears the teeth are inclined wrt the axis of rotation at an angle known as the helix angle. The herringbone gear is essentially a pair of helical gears in which the helix angle is oppositely directed. Spur gears are used in sliding gear blocks, while helical gears are preferred when the gear pairs are permanently in meshing.
Belt transmission:
The belt transmission is used for transmitting rotation between shafts that are located at a considerable distance from each other. It is distinguished by smooth and jerk free rotation which enables its application in high speed machine tools e.g. grinding machines. Belt transmission can be employed for transmitting rotation between parallel and skewed shafts.
The open belt arrangement is employed for transmitting motion between parallel shafts rotating in the same direction. The cross belt arrangement is used when rotation is transmitted between parallel shafts rotating in opposite directions and the quarter-turned arrangement is used for transmitting rotation between skewed shafts.
In machine tool drives flat, V-shaped and round belts are used. Round belts find application in table model machine tools in which torques are of small magnitude. Flat belts are the most versatile as they can be employed in all the three arrangements
For proper functioning of the belt drive, it is essential to provide some mechanism which keeps the belt tight during operation; this increases their cost. Other major drawbacks of the belt transmission are its relatively large dimensions and inability to guarantee constant transmission ratio due tounavoidable slip between the belt and pulleys.
Chain transmission:
The chain transmission is employed for transmitting rotation only between parallel shafts that are located at a considerable distance. The chain transmission consists of a driving sprocket, driven sprocket and chain. Chain transmission is used in machine tools when it is essential to keep the dimension of the drive within reasonable limits and also ensure transmission without slip. The rpm of the driven shaft is determined as.